2/ Historically Brandjacking started on a wrong footing. Here's the wikipedia entry that shows it as an unethical and fraudulent practice. As recently as in 2013, Oxfam Brandjacked Coke's ads to draw public attention to unsustainable business practices.
As e-commerce space grew, fraudulent websites started to hijack popular, growing brands to divert traffic. Terms like Black Hat SEO, Phishin, Domain Kiting, Cybersquatting became more and more common. Here's a 2008 article! It shows how rampant the practice was.
4/ Here's a google ngrams snapshot. The graph shows the increased usage of the word in popular literature and it reached peak usage in 2016. Most of the chatter was on how brands can protect themselves against an onslaught of Brandjacking.
5/ Over the years, the meaning of brandjacking has changed! Here's an article from the Marketingweek that shows that Brandjacking can have positive impact as well, if planned carefully.
6/ It is true that #brandjacking has become easy as It's quite easy to hijack the brand's digital insignia. But this can also be done offline. In retail business imitative packaging is quite common. That concept is explored in this article.